15 October 2010

Colors Published

Our major paper on the colors of Saturn's moons was published last week online at the journal Icarus. I have indeed posted on this before (see last October and February), but as always I have a few new graphics to share.

One of the more interesting features are the broad equatorial blue bands on Mimas (discovered here first) and on Tethys (discovered by Voyager first), which are in fact due to the impact of lots of high energy MeV or greater electrons that travel more slowly in Saturn's magnetic field and appear to be spiraling in "retrograde" into the front side of these satellites. (This is opposite of what happens on Europa due to the different energies involved). What is so amazing is that tiny electrons have the power to alter the surfaces of these satellites. Why the blue (really ultraviolet) brightening is unclear but the subsequent observation that these same areas are colder in the daytime than they should be (observed by my friends on the Cassini CIRS team) adds a key tot he puzzle and suggests that the surface structure is being altered on the microscopic level enough to change the thermal inertia (ability to conduct heat) of the upper centimeter of the surface! Who would have thought. Scientists are looking at this now with new data expected over the next few months.

Enhanced color view of the leading hemisphere of Mimas, showing the large crater Herschel and the broad ultraviolet band across the equator (shown in blue in this RGB rendering)

The second excitement is from Rhea. First the plasma teams observed very odd signatures around Rhea in 2007 which looked rather like the telltale signs of a thin ring around this otherwise heavily cratered satellite about the size of Alaska. Then, looking at stereo images of the craters I noticed an odd blue (really ultraviolet) patch that seemed to be right on the equator. "What a minute," I says to myself "Thats odd." So I made a global map and sure enough the spots went almost all the way around! Only a ring could do that! But then, when the imaging camera was trained to look specifically for a Rhea ring and the next close pass for the plasma instruments happened in 2009, neither time was a ring observed. Hmmm . . . Here we had direct evidence for surface impact onto the surface of small bits from a ring around Rhea (disturbing the dusty coating on the icy surface) and yet the ring turns out to be some sort of as yet unexplained phantom. Well, we don't need the ring to be present today to explain the ultraviolet splotches on the surface. They could have formed a few thousand or million years ago and still exist on the surface today. Probably not much longer than that but thats very young for the Solar System. Its times like these when I really enjoy my job.

Enhanced color medium resolution (2009) view of equatorial region of Rhea from Cassini orbiter. The ring deposits are the dark splotches running east-west along center frame. Turns out these patches don't have the same color shift as those on the leading hemisphere, perhaps due to the presence of E-ring dust on that hemisphere.

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NOTES ABOUT THE DATA

These brief notes are posted to help viewers understand the nature of the topographic data. Voyager, Galileo and Cassini carried imaging cameras but not altimeters. Topography is instead generated using stereo images or low-Sun images (which can be used to calculate slopes and heights). Neither method is perfect and often, as is the case for Miranda, individual stereo pairs must be constructed and then stitched back together to form a global or partial topographic map. This means that the elevations shown are not precise with respect to the center of the body. Height values derived are accurate however with respect to local features. For example, we know the steep cliff on Miranda is about 10 km high top to base but we do not know how high it is with respect to the mean "sea level" on Miranda. This cannot be determined until we return to these places.I will post additional information on the data over the next few days.

Inquiries for the scientific use of the original digital elevation data should be forwarded to schenk@lpi.usra.edu.